An LCD utilizes a twisted nematic liquid-crystal (LC) material supported in a liquid-crystal (LC) module or panel. The LC module includes two glass substrates that sandwich the LC material. Each of the two glass substrates includes transparent electrodes that are used to electrically address locations (image pixels) of the LC module. Two sheets of polarizers sandwich the two glass substrates of the LC module. The two sheets of polarizers are arranged to be cross-polarized. For a reflective LCD that relies on ambient light for its illumination, the LCD includes a reflective surface on the backside of the rear polarizer.
When an image pixel is in the off state, there is no electric field applied to the LC material therein via the transparent electrodes, and the LC material of the image pixel rotates the polarization of light passing therethrough by 90 degrees so that it is transmitted through the LC module and the polarizers. In the on state, the LC material at the image pixel is adjusted, depending on the applied voltage, to vary the rotation of the polarization of the light passing through the LC material from 0 degrees (maximum voltage) to 90 degrees (no voltage). The transmission of the light through the LC module and the polarizers at a given image pixel is therefore determined by the amount of voltage applied by the transparent electrodes. In this configuration, the LCD is transparent in the off state.
Transparent LCDs are difficult to backlight properly, especially when the LCD relies on reflected ambient light for backlighting. A bright light source directly in the line of sight, equal to or larger than the size of the LCD and within a few feet of the LCD, is required for optimum performance. For the best overall viewing experience, the reflective surface that provides the backlighting from the ambient light generally needs to be highly reflective. Present-day LCDs thus work much like a one-way mirror and do not actually have a high degree of transparency, with the most transparent LCDs being about a 16% transparent.